Optimal non surgical treatment of prostate cancer requires dose escalation which is frequently provided by adding a brachytherapy "boost" to a short course of external beam radiotherapy. The hypothesis in this randomized study is that a High Dose Rate (HDR) brachytherapy boost leads to equivalent or better Prostate Specific Antigen (PSA) recurrence-free survival when compared to a Low Dose Rate (LDR) brachytherapy boost and that it is associated with a more favorable toxicity profile and improved quality of life.